1. Field of the Invention
The invention relates to a clock and data recovery circuit (hereafter called CDR), and more particularly to a CDR with built in jitter tolerance test.
2. Description of the Related Art
FIG. 1 is a block diagram of testing of a CDR by means of a tester. Referring to FIG. 1, in jitter tolerance testing, a jitter source 104 and a pattern generator 106 are used to generate a data stream Dj containing jitter and an original data stream DC to be sent to a CDR 102 and a bit error rate (BER) tester 108. The CDR 102 receives the data stream Dj containing jitter and performs clock and data recovery operations to generate a recovery data stream DR and a recovery clock signal CR. Finally, a bit error rate tester 108 compares recovery data stream DR with the original data stream DC to display a bit error rate BER if there is a bit missing or an error occurs. Throughout the specification and claims, the following terms take the meanings explicitly associated herein, unless the context clearly dictates otherwise. The term “bit error rate” refers to the number of bit errors (received by a receiver) divided by the total number of transferred bits. The term “jitter” refers to a deviation of a signal edge from where it should be.
In view of the conventional method that needs an expensive instrument to generate the data stream Dj containing jitter, several jitter measures are then proposed. For example, in a case where a transmit output and a receive input built in the same integrated circuit are connected to each other, providing a data stream containing jitter at the transmit output is used to test the characteristics of a receiver. However, several conventional methods impose jitter on a data stream by increasing additional circuits, thereby resulting in a larger circuit size. In addition, since circuits that impose jitter are generally implemented by analog circuits and the analog circuits vary according to voltages, temperatures and manufacturing processes, the imposed jitter amplitude cannot be estimated accurately.